1. Field of the Invention
The present invention relates to a copying machine for copying images of a document with variable copying concentration.
2. Description of Related Art
When a double-faced document having images at both faces is copied, especially when the document is smaller in thickness, the images at the rear face are likely to be copied during the copying of those on the front face, that is the so-called back image copying may occur.
A method for preventing the above is disclosed in the Japanese Patent Application Laid-Open No. 61-160769 (1986), in which image concentration in the double-faced document copying is adjusted to be brighter than that in the single-faced document copying.
In the prior art, image concentration is also adjusted corresponding to document concentration which is concentration of background of the document. The document concentration is detected on the basis of light reflected from the document. However, some documents being equal in document concentration may be different in light transmittance. The larger the light transmittance, the more often the back image copying occurs. Therefore, even if the image concentration is adjusted corresponding to the document concentration, the back image copying may not be prevented.
Furthermore, in order to avoid the back image copying if the image concentration may be lowered too much, visibility of copy is inferior.